(a) Field of the Invention
The invention relates to a liquid crystal display panel with high cell gap uniformity and a method for precisely controlling the cell gap of a liquid crystal display panel.
(b) Description of the Related Art
A liquid crystal display panel commonly includes two insulating substrates placed opposite to each other with a predetermined cell gap in between. The cell gap uniformity of a liquid crystal display panel is a critical factor that affects display performance. If the cell gap at different locations of a panel is not identical, different regions of the panel may achieve their respective phase differences Δnd to result in Mura defect that considerably lowers display performance of a liquid crystal display, particularly a super-twisted nematic liquid crystal display (STN LCD) where an uneven cell gap is always accompanied by uneven display contrast.
Many factors may affect the uniformity of the cell gap of a display panel. For example, when driving electrodes wire outside the active areas of a panel, their distribution density is often not uniform because of a confined layout space. As the wiring density is not uniform, the spacers that control the cell gap thickness may provide different sustaining forces; hence, the region in which the spacers provide larger sustaining forces has a thicker cell gap while the region in which the spacers provide smaller sustaining forces has a thinner cell gap to result in an uneven cell gap of a display panel. Also, the accumulated tolerance of film thickness for laminated layers or various errors from fabrication processes may cause changes in surface flatness to result in an uneven cell gap.
Hence, there are many conventional designs disclosed to improve the cell gap uniformity of a display panel.
As shown in FIG. 1, a method for controlling a cell gap is disclosed in U.S. Pat. No. 6,712,659. Referring to FIG. 1, when upper and lower substrates 104 and 106 are thermally pressed and assembled together by a press means 102, a detector 108 measures the cell gap thickness at a position designated by a transparent hole 110, and than the detected value is fed to a controller 112 so as to adjust the applied force provided by the press means 102 until the cell gap uniformity is satisfied. However, such design requires an accurate optical instrument for dynamically measuring the cell gap thickness to thus significantly increase the cost. Besides, since only one panel is measured for cell gap thickness at a time according to such design, it is not suitable for mass production.
Further, U.S. Pat. No. 5,629,787 discloses a method for adjusting cell gap during the assembly of two opposite substrates. Referring to FIG. 2, an elastic sheet 202 is provided under an adhesive sealant 204 for adjusting the gap thickness of the assembled upper and lower substrates 206 and 208. However, this method needs a special elastic sheet 202 that has specific surface roughness and fabrication precision and thus is complicated to make. This also increases the fabrication cost and is unfavorable for mass production.
Besides, U.S. Pat. No. 5,757,450 discloses a method for increasing cell gap uniformity by adjusting wiring density of transparent electrodes. As shown in FIG. 3, conventional terminals 304 extending from transparent electrodes 302 often have inclined wiring arrangement, so the transparent electrodes 302 have relatively low wiring density at their edge portions to affect the cell gap uniformity. Hence, according to this method, dummy electrodes 306 are additionally provided at low-wiring-density edge portions to improve the cell gap uniformity. However, this technique cures the uneven cell gap only as a result of uneven electrode wiring density but fails to solve that problem caused by other factors.